1. Field of the Invention
The present invention relates to a motor drive apparatus which supplies drive power to a motor by first converting AC power supplied from an AC power supply to DC power and then converting the DC power to AC power for driving the motor, and more particularly, the invention relates to a motor drive apparatus having a power failure detection unit for determining the presence or absence of a power failure.
2. Description of the Related Art
In a motor drive apparatus for driving motors used in such machines as machine tools, forging presses, injection molding machines, and various kinds of robots, a motor speed, torque, or rotor position command is generated to control the operation of each of the motors provided one for each drive axis. FIG. 8 is a diagram showing the configuration of a conventional motor drive apparatus for driving a plurality of motors. It is to be understood that, throughout the different drawings given herein, the same reference numerals designate component elements having the same functions. The motor drive apparatus 100 includes a 120-degree conduction mode rectifier 11 which converts AC power supplied from a commercial AC three-phase power supply 3 to DC power, and an inverter 12 which converts the DC power output by the 120-degree conduction mode rectifier 11 to AC power of a desired frequency and supplies the AC power as drive power to a motor 2, or which converts AC power regenerated by the motor 2 to DC power, and the motor drive apparatus 100 controls the speed, torque, or rotor position of the motor 2 connected to the AC side of the inverter 12.
For every inverter 12 there are motors 2 provided in order to drive and control the motors 2 by separately supplying drive power to each of the motors 2 provided one for each of the plurality of drive axes. On the other hand, usually only one 120-degree conduction mode rectifier 11 is provided for the plurality of inverters 12 in order to save the cost and installation space of the motor drive apparatus 100.
When the motor is being decelerated under the control of the motor drive apparatus 100, regenerative power is produced by the motor 2. The regenerative power is fed through the inverter 12 back to the 120-degree conduction mode rectifier 11. Since the 120-degree conduction mode rectifier 11 is a relatively low-cost rectifier having a power regeneration function for returning power recovered during motor deceleration to the power supply, and is capable not only of powering operation for converting AC power to DC power but also of regenerative operation for converting DC power back to AC power, the 120-degree conduction mode rectifier 11 can feed the regenerative energy recovered through the inverter 12 back into the AC line connecting to the commercial three-phase AC power supply 3.
In the motor drive apparatus 100, if a power failure occurs on the AC side of the 120-degree conduction mode rectifier 11, the input supply voltage drops, and the motor 2 becomes unable to continue normal operation. To avoid such a situation, it is standard practice to provide a power failure detection unit 52 on the AC side of the 120-degree conduction mode rectifier 11 so that, when the occurrence of a power failure is detected by the power failure detection unit 52, the operation of the motor drive apparatus 100 is switched so as to protect the motor drive apparatus 100, the tool connected to each motor being driven by the motor drive apparatus 100, the object being worked on by the tool, etc. The power failure detection unit 52 detects the presence or absence of a power failure based on the AC voltage detected by an AC voltage detection unit 51. In normal operation when no power failure is detected by the power failure detection unit 52, a host control apparatus 53 sets a switch 55-1 ON and a switch 55-2 OFF in a switch unit 55. With this setting, the AC power from the commercial three-phase AC power supply 3 is converted by the 120-degree conduction mode rectifier 11 into DC power which is supplied to each inverter 12. The host control apparatus 53 sends a motor drive command to each inverter 12 and controls the DC-to-AC conversion operation of the inverter 12 (more specifically, the switching operation of the switching device in the inverter 12) so that desired AC power is output from the inverter 12. Since the AC power output from the inverter 12 is used as the drive power to drive the motor 2, the speed, torque, or rotor position of the motor 2 connected to the AC side of the inverter 12 can be controlled by controlling the AC power to be output from the inverter 12. On the other hand, when a power failure is detected by the power failure detection unit 52 based on the AC voltage detected by the AC voltage detection unit 51, the host control apparatus 53 sets the switch 55-1 OFF and the switch 55-2 ON in the switch unit 55. As a result, DC power stored in a power storage device 54 is supplied to each inverter 12. The host control apparatus 53 transmits to each inverter 12 a motor drive command for protecting the motor drive apparatus 100, the tool connected to each motor being driven by the motor drive apparatus 100, and the object being worked on by the tool.
FIG. 9 is a circuit diagram for explaining currents flowing in the 120-degree conduction mode rectifier during the powering operation by the conventional motor drive apparatus. In the illustrated example, each inverter 12 connected to the 120-degree conduction mode rectifier 11 via a smoothing capacitor C is omitted from illustration. When supplying drive power to the motor, the 120-degree conduction mode rectifier 11 turns off the switches SWR1, SWR2, SWS1, SWS2, SWT1, and SWT2 on all of the R-, S-, and T-phases. This operation mode will hereinafter be referred to as “powering operation of the 120-degree conduction mode rectifier.” During the powering operation of the 120-degree conduction mode rectifier 11, if there is current from the commercial three-phase AC power supply 3, for example, on the R-phase in a given cycle, this AC current is output to the DC side through a diode DR1 on the R-phase, and the current from the DC side returns to the commercial three-phase AC power supply 3 through a diode DT2 on the T-phase. The same is applied for other phase currents that can occur at other times.
FIG. 10 is a circuit diagram for explaining currents flowing in the 120-degree conduction mode rectifier during the regenerative operation by the conventional motor drive apparatus. In the illustrated example, each inverter 12 connected to the 120-degree conduction mode rectifier 11 via the smoothing capacitor C is omitted from illustration. The 120-degree conduction mode rectifier 11 controls the on/off operations of the switches SWR1, SWR2, SWS1, SWS2, SWT1, and SWT2 on the R-, S-, and T-phases as needed so that the regenerative power recovered from the motor 2 via the inverter is converted to AC power and returned to the commercial three-phase AC power supply 3. This operation mode will hereinafter be referred to as “regenerative operation of the 120-degree conduction mode rectifier.”During the regenerative operation of the 120-degree conduction mode rectifier 11, the R-phase switch SWR1 and the T-phase switch SWT2 is turned on at a given time and the other switches SWR2, SWS1, SWS2, and SWT1 are turned off. As a result, the current that occurs when the regenerative power from the motor 2 at the given time is fed back to the 120-degree conduction mode rectifier 11 via the inverter is passed via the R-phase switch SWR1 back to the commercial three-phase AC power supply 3, and the current from the commercial three-phase AC power supply 3 is supplied to the DC side via the T-phase switch SWT2. The same is applied for R-phase and T-phase currents that can occur at other times.
As a method for determining a power failure in the motor drive apparatus, Japanese Unexamined Patent Publication No. 2006-14546, for example, discloses a method that converts the three-phase AC input voltage into a voltage vector in a two-phase coordinate system, calculates the amplitude of the input voltage from the amplitude of the vector, and determines that a power failure has occurred when the condition in which the amplitude value is lower than a predetermined reference voltage value has continued for a predetermined reference period of time. FIG. 11 is a circuit diagram showing a power failure detection unit for determining the presence or absence of a power failure based on the amplitude of the three-phase AC input voltage. The voltage from the commercial three-phase AC power supply 3 is detected by an AC voltage detection unit 122, and the amplitude of the voltage is calculated by a voltage amplitude detection unit 126. When the condition in which the value of the voltage amplitude calculated by the voltage amplitude detection unit 126 is lower than a predetermined reference voltage value has continued for a predetermined reference period of time, the power failure detection unit 125 determines that a power failure has occurred.
As another method for determining a power failure in the motor drive apparatus, Japanese Unexamined Patent Publication Nos. H06-169501 and H06-189411, for example, propose a method that calculates the frequency of the input voltage (power supply frequency) and determines that a power failure has occurred when the frequency calculated falls outside a predetermined range. FIG. 12 is a circuit diagram showing a power failure detection unit for determining the presence or absence of a power failure based on the frequency of the three-phase AC input voltage, and FIG. 13 is a diagram for explaining the basic concept of the power failure detection unit that determines the presence or absence of a power failure based on the frequency of the three-phase AC input voltage. The voltage from the commercial three-phase AC power supply 3 is detected by an AC voltage detection unit 122, and the frequency is calculated by a frequency calculation unit 123. The power failure detection unit 125 determines, based on the frequency calculated by the frequency calculation unit 123, the presence or absence of a power failure on the AC side of the 120-degree conduction mode rectifier 11. For example, a predetermined range of frequencies centered about the commercial power supply frequency (50 Hz or 60 Hz) is predefined as a normal frequency range, as shown in FIG. 13. As long as there is no power failure on the AC side of the 120-degree conduction mode rectifier 11, the frequency calculated by the frequency calculation unit 123 shows a substantially constant value, but when a power failure occurs, for example, at time A in FIG. 13, the frequency being calculated by the frequency calculation unit 123 begins to fluctuate. In the illustrated example, the frequency is shown as gradually increasing after the occurrence of the power failure but, depending on the situation of the power failure, the frequency may decrease, oscillate, or diverge. Then, when the frequency deviates outside the normal frequency range at time B, the power failure detection unit 125 determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier 11.
With the method that detects the occurrence of a power failure based on the amplitude of the three-phase AC input voltage, such as disclosed in the above-cited Japanese Unexamined Patent Publication No. 2006-14546, when a 120-degree conduction mode rectifier is used as the converter for converting the AC power supplied from the commercial three-phase AC power supply to DC power, if a power failure occurs during the regenerative operation of the 120-degree conduction mode rectifier, it is not possible to detect the power failure. FIGS. 14 and 15 are diagrams for explaining the problem associated with the power failure detection performed based on the voltage amplitude when the 120-degree conduction mode rectifier is used: FIG. 14 shows the power failure detection during the powering operation of the 120-degree conduction mode rectifier, and FIG. 15 shows the power failure detection during the regenerative operation of the 120-degree conduction mode rectifier. In the illustrated example, each inverter 12 connected to the 120-degree conduction mode rectifier 11 via the smoothing capacitor C is omitted from illustration.
As shown in FIG. 14, when a power failure occurs on the AC side of the 120-degree conduction mode rectifier 11 during the powering operation of the 120-degree conduction mode rectifier 11 by the motor drive apparatus, since the switches SWR1, SWR2, SWS1, SWS2, SWT1, and SWT2 in the 120-degree conduction mode rectifier 11 are all OFF, the voltage that the voltage detection unit 122 detects on the AC side of the 120-degree conduction mode rectifier 11 drops to almost zero, upon detection of which it can be determined that a power failure has occurred on the AC side.
However, as shown in FIG. 15, when a power failure occurs on the AC side of the 120-degree conduction mode rectifier 11 during the regenerative operation of the 120-degree conduction mode rectifier 11 by the motor drive apparatus, since the DC output voltage appears at the voltage detection unit 122 through the ON switches, the voltage amplitude does not drop, and therefore, it is not possible to detect the power failure. In the illustrated example, since the switches SWR1 and SWT2 are ON, the DC output voltage appears on the AC side of the 120-degree conduction mode rectifier 11 through the switches SWR1 and SWT2. As a result, even when a power failure has actually occurred, it is not possible to detect the occurrence of the power failure, because the amplitude of the voltage on the AC side of the 120-degree conduction mode rectifier 11 does not drop.
On the other hand, in the case of the method that determines the presence or absence of a power failure based on the frequency of the three-phase AC input voltage, such as disclosed in the above-cited Japanese Unexamined Patent Publication Nos. H06-169501 and H06-189411, unlike the method that determines the presence or absence of a power failure based on the amplitude of the three-phase AC input voltage, the occurrence of a power failure at the AC power supply side can be detected even during the regenerative operation of the 120-degree conduction mode rectifier. However, if the normal frequency range is narrowed in order to advance the power failure detection timing, the possibility of an erroneous power failure detection increases; conversely, if the normal frequency range is widened in order to prevent an erroneous power failure detection, the time taken to detect the occurrence of a power failure increases, resulting in an inability to protect the motor drive apparatus, the tool connected to each motor being driven by the motor drive apparatus, the object being worked on by the tool, etc. FIGS. 16 and 17 are diagrams for explaining the problem associated with the power failure detection performed based on the voltage frequency when the 120-degree conduction mode rectifier is used: FIG. 16 shows the power failure detection when the normal frequency range is narrowed, and FIG. 17 shows the power failure detection when the normal frequency range is widened.
When the normal frequency range is narrowed in order to advance the power failure detection timing, as shown in FIG. 16, if a power failure occurs, for example, at time D in FIG. 16, the frequency being calculated by the frequency calculation unit 123 begins to fluctuate (increase in the example of FIG. 16), and when the frequency deviates outside the normal frequency range at time E, the power failure detection unit 125 determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier 11. The narrower the normal frequency range, the shorter the time taken to detect abnormality in frequency, i.e., the time taken from the moment a power failure occurs (time D) to the moment the power failure is detected (time E). The fluctuation of the power frequency that occurs when a power failure occurs during the regenerative operation of the 120-degree conduction mode rectifier depends on the load of the motor drive apparatus and the impedance, and there are cases where the fluctuation is mild; therefore, from the standpoint of detecting a power failure early, it is desirable that the detection sensitivity to the frequency fluctuation be higher. However, the actual frequency (50 Hz or 60 Hz) of the commercial three-phase AC power supply 3 more or less fluctuates even during normal operation and, even when there is no power failure, the frequency may deviate outside the normal frequency range, as indicated at C in FIG. 16, depending on the way the frequency fluctuates. For example, when the AC power supply is a small-sized power supply implemented in a distributed power supply system, the frequency fluctuation is large, and the possibility of an erroneous power failure detection increases.
On the other hand, when the normal frequency range is widened in order to prevent an erroneous power failure detection, as shown in FIG. 17, if a power failure occurs, for example, at time F in FIG. 17, the frequency being calculated by the frequency calculation unit 123 begins to fluctuate (increase in the example of FIG. 17), but the length of time that elapses until reaching time G at which the power failure detection unit 125 determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier 11 increases. This increases the possibility that the motor drive apparatus, the tool connected to each motor being driven by the motor drive apparatus, the object being worked on by the tool, etc., may not be protected before damage is caused.